Generally, a cast slab is made in such a manner that molten steel received in a mold is cooled through a cooling unit, which is shown in FIG. 1. A continuously cast slab 10 is cooled while passing through at least one segment 20 and then progresses to a following process. When the cast slab is rolled into a thick steel plate, the defect of the cast slab may remain even after the rolling, which may cause inferiority. This defect may be center segregation and porosity, for examples. The center segregation occurs by the flow of solute enriched in the residual molten steel in the final stage of solidification when a slab is continuously cast. The major factor of this flow is cast slab bulging and solidification shrinkage of residual molten steel. However, the center segregation is most influenced by the flow of residual molten steel caused by the solidification shrinkage near a solidification end point, except for the cast slab bulging caused by mechanical factors. That is, if residual molten steel with enriched solute (referred to as so-called ‘solute-enriched molten steel’) is collected in a solidification shrinkage region near the solidification end point in the continuous casting process, this becomes center segregation. If the solidification shrinkage region is not filled but remains as a space, it becomes center porosity.
A representative technique for decreasing defects such as center segregation and porosity is soft reduction process. The soft reduction is to endow reduction force to a cast slab 10 by the segment 20 during a continuous casting process. The cast slab 10 is reduced as much as the solidification shrinkage at the end of solidification stage to physically compress a shrinkage cavity, whereby solute-enriched molten steel existing between columnar dendrites by solidification shrinkage is restrained from being introduced into a thickness center area of the cast slab to thereby improve center segregation of the cast slab.
FIG. 2 is a sectional view showing a cast slab in a casting direction during a continuous casting process.
The essence of the above soft reduction is that weak pressure is applied to a solid/liquid coexisting region, so called a mushy zone (having a solid fraction from 0.3˜0.4 to 0.7˜0.8), where center segregation is formed by residual molten steel collected in and around a shrinkage cavity, which is formed during a solidification process. However, the soft reduction applied at the point where a shrinkage cavity is formed has the following problems.
First, the soft reduction technique allows a small reduction amount (a total reduction amount: 3˜5 mm), and allows equiaxed dendrites to be easily formed at the thickness center portion of the cast slab at the end of solidification stage. In this case, the reduction force at a surface region of the cast slab is not easily transmitted to the thickness center region of the cast slab (a reduction efficiency is about 20%), so that the shrinkage cavity is not fully compressed. Accordingly, residual molten steel in which a solute is enriched may be collected in the partially uncompressed shrinkage hole to form a small center segregation, or a porosity remains in the thickness center portion of the cast slab. Also, the continuous cast slab causes solidification irregularity in a slab width direction during the casting process, and if the cast slab is softly reduced, the reduction force is changed depending on a position in the slab width direction, so that it is difficult to uniformly compress a shrinkage cavity over the entire cast slab to eliminate defects. In addition, the reduction force does not reach a center portion of the cast slab spaced apart from an edge of the cast slab by a predetermined distance due to the influence of the solidified layers formed at marginal portions of the cast slab. As a result, the interior quality is greatly changed in a slab width direction, and center segregation or center porosity happens near the slab center portion, so that defects occur intensively at a local portion of the thick steel plate.
Due to the above reasons, the existing soft reduction has a limitation in controlling center segregation, when it is used singly. For improvement, the following methods have been proposed.
First, there has been proposed a method wherein after applying soft reduction to a region having a solid fraction from 0.3˜0.4 to 0.7˜0.8, at least one pair of additional rolls are installed at the end location of solidification stage corresponding to the solid fraction of 0.8˜1.0 and heavy rolling is performed. In this method, the existing soft reduction is applied as it was, and then the following region of the cast slab is rolled using rolls, where center segregation is in the same level as the existing soft reduction. However, in this method, when solidification irregularity occurs in a slab width direction, interior quality control is difficult in the width direction. In addition, equipment remodeling for installing rolls is required, and the final solidification portion should be identically arranged at a location where the rolls are installed. Thus, this method has a fundamental limitation in that it cannot cope with a location of solidification finishing point, which varies according to a change in slab width or other work conditions such as casting speed change.
In the conventional technique as mentioned above, a thick cast slab at the end of solidification stage is heavily rolled using additional rolls, so that since great reduction force is required, rolls should be essentially installed. When a cast slab is reduced by means of rolls, both end sides of the cast slab are already in a fully solidified solid state. Thus, when reduction is executed using the rolls, the fully solidified solid layer is reduced, whereby the great reduction force is required such that the reduction force can be transmitted up to the center of the cast slab.
In addition, since a great reduction of 3 to 15 mm is applied to a cast slab (the entire of which is substantially solidified) in order to reduce porosities occurring in the slab center portion with a solid fraction of 0.8 or above, the extremely great reduction force is required. Accordingly, if rolls which are not reinforced are used for applying the great reduction force, the rolls may be broken. Thus, there has been proposed a technique for reinforcing rigidity of rolls by increasing a diameter of rolls from 300 mm to 450 mm, as a countermeasure. However, this method cannot also avoid deterioration of interior quality (occurrence of interior cracks) of the continuous cast slab according to the increase of roll pitch of the continuous casting machine. That is, the bulging that gives a great influence on occurrence of interior cracks and center segregation of the continuous cast slab is proportional to the fourth power of the roll pitch of the continuous casting machine, and the rolls of the continuous casting machine is substituted with rolls with a great diameter as mentioned above, so that the quality of the continuous cast slab is inevitably deteriorated under the casting conditions using the rolls for the continuous casting machine since a casting speed is changed when producing different kinds of general steels using the same continuous casting machine. In addition, if the cast slab is heavily rolled using the rolls after the soft reduction is applied, the center segregation is more enriched. That is, even when the soft reduction is applied, the center segregation remains in the thickness center portion to some extent, and if the cast slab is heavily rolled using the rolls in this state, the center segregation portion is also rolled, thereby increasing the degree of solute enrichment in the center segregation portion and also changing the residual shape into a sharp linear form. In this case, properties of the rolled steel are easily deteriorated after the rolling work.